This invention is directed to electronic organs utilizing a plurality of integrated circuit chips, each chip time division multiplexing selected time constant signals from a switched capacitor time constant generator for two alphabetic notes over the entire frequency range of a spinet or for one alphabetic note over the entire frequency range of the manuals and pedals of a console organ thereby reducing the amount of circuitry and interconnection wiring; and, more particularly, to an integrated circuit package for generating time division multiplexed keying envelope signals for such time multiplexed electronic organs at a high rate of speed to maintain the frequency response characteristics.
Electronic organs are of two general varieties, the synthesis organ and the formant organ. In the synthesis organ, musical tones are synthesized by mixing properly scaled sine waves having frequencies representative of the fundamental and various harmonics of the tones to be synthesized. In formant electronic organs, so-called "bright waves" or signals which are rich in harmonic content including a fundamental frequency and a full complement of harmonics are filtered by formant filter circuits to remove unwanted harmonics and alter the harmonic balance of these complex signals to arrive at desirable tone signals.
Both types of electronic organs require keys or pedals to generate signals indicative of tones to be played by the organ. Various types of keying circuits, such as those disclosed in U.S. Pat. No. 3,636,231, are provided and controlled by the individual key or pedal signals to pass time constants signals which, in turn, activate tone generator circuits to generate the desired tones. In both synthesis and formant organs, it is advantageous to provide an arrangement for controlling the tone envelope, i.e., the rate of attack and decay of the tone signal, to avoid transients which introduce noise and also to achieve various desirable special effects. To achieve this purpose, each playing key drives a time constant circuit to impose upon the keying signal a defined envelope. The keying envelope signals which are generated by the organ playing keys or pedals in conjunction with the time constant circuits activate the keying circuits to provide such tone envelopes.
It is well known in the prior art to provide time division multiplexing of various information provided to the keying circuits, for example, draw-bar information, various control switches or tabs and the envelope keying signals. This information is divided into discrete repetitive time slots and provided to the keying circuits. A corresponding demultiplexer is provided at the output of the tone generator to distribute the audio signals among various filter circuits to ultimately provide the requested tones.
The standard in the prior art of electronic organs has been to generate the time constants necessary for the keying envelopes by utilizing the charging and discharging characteristics of a capacitor as part of a resistor capacitor (RC) time constant circuit. Recently developed switched capacitor techniques have overcome a major obstacle to the integration of time constant circuits, i.e., the implementation of resistors, by simulating resistors with high speed switched capacitors. This approach eliminates the necessity for precise integrated resistor values previously obtained only by hybrid devices that require costly trimming procedures.
Although the integration of time constant circuits utilizing switched capacitor techniques has greatly reduced the number of components and the size of those components included in electronic organs, the interconnection of the integrated time constant circuits into the remaining circuitry requires considerable wiring and does not take maximum advantage of the reduced size of the integrated time constant circuits.